The purpose of the proposed research is to make new high-resolution measurements of basic elastic and contractile properties of cardiac muscle in order to better understand the interaction of these properties in contraction of cardiac muscle. For twenty years the development of the muscle twitch has been explained in terms of active contractile elements delivering force through passive elastic elements, but the model of A.V. Hill based on such reasoning has quantitative limitations. Alternate methods for studying heart muscle have been suggested which involve direct alterations in muscle length instead of the classical procedure of altering force and measuring the resulting length changes. Important contradictions have been found suggesting that muscle elasticity may vary substantially during the course of the twitch, thus both contradicting the traditional passive description of muscle elasticity, and introducing errors into previous methods of measuring contractile properties. The present research proposes to measure muscle elasticity and contractility with new high-resolution equipment by a much more detailed and systematic procedure giving special attention to previously unexplored effects of measurement parameters such as release velocity, duration, and amplitude, choice of force or length as independent variable, and real-time as compared with extrapolated force and length data. Such differences in measurement technique may help to explain contradictory data on elasticity relative to time. The proposed study will provide insight into contractile element properties. These investigations are vital to an improved understanding of the underlying events in normal and abnormal contractions of the heart.